1. Field of the Invention
The present invention relates to an optical branching circuit and device connected to a light output end face of an optical waveguide circuit or an optical branching circuit and for branching input light.
2. Description of the Related Art
A general optical branching circuit is comprised of an input optical waveguide, a taper-shaped transition region (tapered optical waveguide) for adiabatically changing a light distribution, and respective branching optical waveguides.
Although the optical branching circuit is designed so that optical powers propagated to the respective branching optical waveguides have a desired value (branching ratio), a measured value of an actually fabricated optical branching circuit often becomes a value different from a calculated branching ratio. For example, an optical power propagated to each of the branching optical waveguides is determined by the overlap degree of a light electric field distribution at both sides of a boundary between the tapered optical waveguide and the branching optical waveguide.
In the case where two branching optical waveguides are arranged symmetrically with each other with respect to the center of the width of the light output end face of the tapered optical waveguide, in order to obtain the one-to-one branching ratio, it is necessary that the light electric field distribution at the tapered optical waveguide side of the boundary becomes symmetrical with respect to the center of the width of the light output end face of the tapered optical waveguide. On the other hand, in order to distribute optical powers having different intensities to two branching optical waveguides by varying the branching ratio, it conventionally becomes possible by making a set of branching optical waveguides offset (shifted) at the boundary with the tapered optical waveguide in the direction parallel to the light output end face of the tapered optical waveguide.
By the way, in an actual optical branching circuit, there has been a problem that even if the shape is the same, variation in branching characteristics occurs. This is noticeable in a case where another optical circuit is connected to a front stage of the optical branching circuit. For example, in the case where the front stage is an optical fiber connection circuit, there is a case where light not coupled with a waveguide due to mode mismatching or the like mixes in the optical branching circuit along the input optical waveguide, and causes a characteristic different from a design. Besides, in the case where an optical branching circuit is connected to the front stage as well, there has been a problem that light generated at a branching portion and not coupled with the waveguide mixes in the latter stage optical branching circuit along the latter stage input optical waveguide, and causes similar variation in characteristics which can not be expected in design.
One of the features of the present invention is to provide an optical branching circuit and device with less branching ratio variation.
The above feature is achieved by an optical branching circuit characterized by comprising an input optical waveguide for outputting input light inputted from a light input end face to a light output end face, a tapered optical waveguide disposed to be shifted with respect to the input optical waveguide at the light output end face, and branching optical waveguides connected to a light output end face of the tapered optical waveguide at a predetermined branching angle.
The above optical branching circuit of the invention is characterized in that the tapered optical waveguide is disposed to be shifted by a predetermined offset amount relative to the input optical waveguide in a direction orthogonal to a center axis of the input optical waveguide.
The above optical branching circuit of the invention is characterized in that the predetermined offset amount is determined based on a wavelength of the input light.
The above feature is achieved by an optical branching device characterized by comprising plural optical branching circuits, each being the above optical branching circuit of the invention, connected in cascade.